Alas, the device tree doesn't match power domains at all.
Back when pmf was being designed, I strongly urged that it
include the concept of power domains under which device could
be grouped.

As long as the tree is flat, it doesn't matter at all.
If it isn't the problem is interaction of different buses and for that
you might end up with a heavily MD system.

While omap only a few buses, unrelated devices are grouped together in
power domains.
The problem is that on embedded systems, this type of power mgmt is very
common and should not require MD hooks. The PM framework should handle
this out of the box.